The present invention relates to an RF probe for a Magnetic Resonance Imaging (hereinafter, referred to as "MRI" for short, when applicable) apparatus in a vertical magnetic system which serves to detect an NMR (nuclear magnetic resonance) signal sent from hydrogen, phosphorus or the like contained in a subject to image the distribution of density, the distribution of a relaxation time, and the like of the nuclei, and more particularly to an RF probe for an MRI apparatus which solves a problem in that a signal-to-noise (S/N) ratio of a multiple-coil is decreased in a deeper detection region.
In an MRI apparatus, the inspection and the photographing of a subject have heretofore been performed using various kinds of RF coils for the head and various kinds of RF coils for the belly which are arranged so as to surround a part of interest of the subject (e.g., a human being), a surface RF coil which is hardly influenced by the motion of the heart or the like, and the like.
Although the above-mentioned surface coil has higher sensitivity than that of the coil for the head and the coil for the belly, its field of view is necessarily limited. As a result, there arises a problem in that when a wide region of the body such as the spine is inspected, the photographing must be performed by several times by moving the surface coil, and therefore, this inspection employing the surface coil needs too much time.
On the other hand, there has been proposed a method in which a plurality of surface coils are arranged in such a way that they are suitably overlapped so as not to be mutually coupled to the surface coils adjacent thereto, and NMR signals which are received by the surface coils are composed thereby to widen substantially the field of view, i.e., a multiple-coil for a horizontal magnetic field MRI. Incidentally, the principle of this method is described in Publication of the Translation of International Patent Application No. 1990/500,175, JP-A-2-13432, and Magnetic Resonance in Medicine, Vol. 16, pp. 192-225 (1990) for example.
In the multiple-coil shown in the above-mentioned prior art technology, the field of view is widened and a high S/N ratio is obtained in a shallower detection region (near the coil). However, there arises a problem in that the S/N ratio is remarkably decreased in a deeper detection region and thus the S/N ratio in the deeper detection region is poor as compared with the coil for the belly.
This situation will hereinbelow be concretely described with reference to FIG. 1. In the above-mentioned multiple-coil, the field of view is widened, and thus, in a parallel conductor part at the center of the coil, the characteristics of sensitivity of a depth direction (Z-axis direction) as indicated by a of FIG. 1 are shown. On the other hand, the coil for the belly which is arranged so as to sorround the subject shows the characteristics of sensitivity of the depth direction as indicated by b of FIG. 1. Thus, in the above-mentioned multiple-coil, although a higher S/N ratio is obtained in the shallower detection region, in the deeper detection region, the S/N ratio is decreased to be smaller than that of the coil for the belly. If this problem can be solved, the measurement region by the surface coil can be widened.
On the other hand, as for a method of improving the reception sensitivity of the detection coil, there has been used a QD (Quadrature Detection) method which is described in Journal of Magnetic Resonance, Vol. 54, pp. 324-327 (1983). This method is a method wherein two coils for generating respective magnetic fields perpendicularly intersecting each other are provided to detect two NMR signals perpendicularly intersecting each other, and a phase of one detection signal is shifted by 90 degrees to compose the two detection signals, thereby to detect the composite signal at high sensitivity. According to this method, in the case where the intensity of the signal sent from one coil is equal to that of the signal sent from the other coil perpendicularly intersecting the one coil, the sensitivity is ideally improved by .sqroot.2 times as compared with the case of the detection of a single signal. However, in the above-mentioned method, QD of the multiple-coil is not taken into consideration.
An example in which the QD method is applied to the multiple-coil for an MRI apparatus in a horizontal magnetic system is described in SMRM "Quadrature Switchable Spine Arry Coils", p.952, 1989.
However, in the case where the QD method is directly applied to the MRI apparatus in the vertical magnetic system, the wide field of view required for the diagnosis cannot be photographed at high sensitivity, and thus, there is no availability.